Blood from the pituitary stalk was collected from the rhesus monkey and analyzed for LRF by radioimmunoassay. During the ovulatory surge of LH, high levels of LRF were found whereas during the follicular phase of the menstrual cycle when LH levels were low, low LRF levels were found. This finding provides an affirmative answer to the longstanding question of whether the ovulatory surge of LH secretion is stimulated by an antecedent rise in hypothalamic LRF secretion. The distribution of LRF in the hypothalamus was also determined. The highest amounts were found in the median eminence, as expected. However, large amounts were also present in the posterior pituitary gland and smaller amounts in the supraoptic crest (organum vasculosum of the lamina terminalis). To investigate the role of LRF these latter organs, they were ablated and the effects on LH secretion determined. No major deficits in LH secretion were noted, all animals eventually returning to regular menstrual cycles accompanied by ovulation. These perplexing results suggest that LRF has a neural regulating role in addition to its well-established role of stimulating LH secretion. BIBLIOGRAPHIC REFERENCES: Neill, J.D., R.A. Dailey, R.C. Tsou, J.M. Patton and G.T. Tindall. Regulation of the Monkey Menstrual Cycle. In: Regulation of Ovulation in the Human, P. Crosignani (ed.), Academic Press, New York, 1976 (in press). Neill, J.D., J.M. Patton, R.A. Dailey, R.C. Tsou and G.T. Tindall. On the Regions of the Brain Regulating LH Secretion in the Rhesus Monkey. 1976 Proceedings of the Endocrine Society, p. 86 (Abstract).